In the technology of industrial process measurements and automation, especially also in connection with the automation of chemical or technical-method processes and/or the control of industrial plants, measuring devices installed near to the process, so-called field devices, are used for locally producing measured-value signals as analog or digital representations of process variables. Likewise, field devices can be embodied as adjusting devices for varying one or more of such process variables and, in such respect, actively guiding the flow of the process. Such process variables to be registered, or adjusted, as the case may be, include, for example, and as can also be perceived from the cited state of the art, mass flow rate, density, viscosity, fill level, limit level, pressure, temperature, or the like, of a liquid, powdered, vaporous or gaseous medium, conveyed, or stored, as the case may be, in a corresponding containment, such as e.g. a pipeline or a tank. Additional examples for such field devices, which are known, per se, to those skilled in the art, are described extensively and in detail in WO-A 05/040735, WO-A 04/048905, WO-A 03/048874, WO-A 02/45045, WO-A 02/103327, WO-A 02/086426, WO-A 01/02816, WO-A 00/48157, WO-A 00/36379, WO-A 00/14485, WO-A 95/16897, WO-A 88/02853, WO-A 88/02476, U.S. Pat. Nos. 7,200,503, 7,004,191, 6,932,098, 6,799,476, 6,776,053, 6,769,301, 6,577,989, 6,662,120, 6,574,515, 6,535,161, 6,512,358, 6,487,507, 6,480,131, 6,476,522, 6,397,683, 6,352,000, 6,311,136, 6,285,094, 6,269,701, 6,236,322, 6,140,940, 6,014,100, 6,006,609, 5,959,372, 5,796,011, 5,742,225, 5,687,100, 5,672,975, 5,604,685, 5,535,243, 5,469,748, 5,416,723, 5,363,341, 5,359,881, 5,231,884, 5,207,101, 5,131,279, 5,068,592, 5,065,152, 5,052,230, 4,926,340, 4,850,213, 4,768,384, 4,716,770, 4,656,353, 4,617,607, 4,594,584, 4,574,328, 4,524,610, 4,468,971, 4,317,116, 4,308,754, 3,878,725, 3,764,880, EP-A 1 158 289, EP-A 1 147 463, EP-A 1 058 093, EP-A 984 248, EP-A 591 926, EP-A 525 920, or EP-A 415 655, DE-A 44 12 388 or DE-A 39 34 007. The field devices disclosed therein are, in each case, fed by an external, electrical energy supply, which provides a supply voltage and a supply current driven thereby, flowing through the electronics of the field devices.
For the case in which the field device serves as a measuring device, it additionally contains an appropriate physical-to-electrical, or chemical-to-electrical, measurement pickup, or transducer, for electrically registering the relevant process variables. Such pickup is, most often, inserted in the wall of the containment carrying the medium or into the course of a line, for instance a pipeline, conveying the medium, and serves to produce a measurement signal, especially an electrical measurement signal, representing the primarily, registered process variable as accurately as possible. For processing the measurement signal, the measurement pickup is, in turn, connected with the operating and evaluating circuit provided in the field-device electronics and serving especially for a further processing or evaluation of the at least one measurement signal. In a large number of such field devices, the measurement pickup is additionally so actuated by a driving signal generated, at least at times, by the operating and evaluating circuit, that the pickup interacts at least directly with the medium in a manner suitable for the measurement or, alternatively, essentially directly with the medium via an appropriate probe, in order to provoke reactions reflecting the parameter to be registered. The driving signal can, in such case, be controlled, for example with respect to an electrical current level, a voltage level and/or a frequency. Examples of such active measurement pickups, thus measurement pickups appropriately converting an electric driving signal in the medium, are, especially, flow measurement pickups serving for the measurement of media flowing at least at times. The pickups utilize at least one coil actuated by the driving signal to produce a magnetic field, or at least one ultrasound emitter actuated by the driving signal, or a fill level, and/or limit level, pickup serving for measuring and/or monitoring fill levels in a container, such as e.g. microwave antennas, Goubau lines, thus a waveguide for acoustic or electromagnetic surface waves, vibrating immersion elements, or the like.
For accommodating the field-device electronics, field devices of the described kind further include an electronics housing, which, as e.g. disclosed in U.S. Pat. No. 6,397,683 or WO-A 00/36379, can be situated remotely from the field device and connected therewith only via a flexible cable, or which, as shown e.g. also in EP-A 903 651 or EP-A 1 008 836, is arranged directly on the measurement pickup or in, or on, a measurement pickup housing separately housing the measurement pickup. Often, the electronics housing then serves, as shown, for example, in EP-A 984 248, U.S. Pat. No. 4,594,584, U.S. Pat. No. 4,716,770, or U.S. Pat. No. 6,352,000, also to accommodate some mechanical components of the measurement pickup, such as e.g. membrane, rod, shell or tubular, deforming or vibrating members deforming during operation, under the influence of mechanical forces; compare, in this connection, also the above-mentioned U.S. Pat. No. 6,352,000. Field devices of the described kind are, furthermore, usually connected together and/or with appropriate process control computers via a data transmission system connected to the field-device electronics. The field devices transmit their measured value signals to such locations e.g. via (4 mA to 20 mA)-current loops and/or via digital data bus and/or receive operating data and/or control commands in corresponding manner. Serving as data transmission systems here are especially fieldbus systems, such as e.g. PROFIBUS-PA, FOUNDATION fieldbus, as well as the corresponding transmission protocols. By means of the process control computers, the transmitted measured value signals can be processed further and visualized as corresponding measurement results e.g. on monitors and/or converted into control signals for other field devices embodied as actuators, e.g. in the form of solenoid-controlled valves, electric motors, etc.
Furthermore, a large number of field devices of the described kind, especially also measuring field-devices, are electrically so designed that they satisfy the requirements for intrinsic explosion-safety. Accordingly, the field devices are operated with such a low electrical power that, for lack of reaching ignition conditions, sparks or arcs can electrically not be released. Intrinsically safe explosion protection is, for example, given according to the European Standards EN 50 014 and EN 50 020, when electronic apparatuses are so constructed that they satisfy the ignition protection classification “Intrinsic Safety (Ex-i)” defined therein. According to this ignition protection classification, thus electrical currents, voltages and powers occurring in the field device are, at all times, not permitted to exceed specified current, voltage and power, limit values. These three limit values are so selected that, in the case of malfunction, or in the case of a short circuit, the maximally released amount of energy is not sufficient to produce an ignition-capable spark. Usually, in the case of intrinsically safe, field devices, the electrical power is not permitted to exceed 1 W (=watt). The voltage can be kept below the specified limit values e.g. by Z-diodes, the current e.g. by resistors, and the power by appropriate combinations of voltage- and current-limiting components.
In the case of modern field devices, these are often so-called two-wire field devices, thus field devices in the case of which the field-device electronics is electrically connected with the external, electrical energy supply solely via a single pair of electrical lines and in the case of which the field-device electronics also transmits the instantaneous measured value via the single pair of electrical lines to an evaluation unit provided in the external, electrical energy supply and/or electrically coupled therewith. The field-device electronics includes, in such case, always an electrical current controller for setting and/or modulating, especially clocking, such as strobing, triggering or firing, the supply current, an internal operating and evaluating circuit for controlling the field device, as well as an internal supply circuit lying at an internal input voltage of the field-device electronics derived from the supply voltage, feeding the internal operating and evaluating circuit and having at least one useful-voltage controller, e.g. regulator, flowed through by a variable electrical current component of the supply current and providing an internal useful voltage in the field-device electronics which is regulated, or controlled, to be essentially constant at a predeterminable voltage level. Examples of such two-wire field devices, especially two-wire measuring devices or two-wire actuators can be found in, among others, WO-A 05/040735, WO-A 04/048905, WO-A 03/048874, WO-A 02/45045, WO-A 02/103327, WO-A 00/48157, WO-A 00/26739, U.S. Pat. Nos. 7,200,503, 6,799,476, 6,577,989, 6,662,120, 6,574,515, 6,535,161, 6,512,358, 6,480,131, 6,311,136, 6,285,094, 6,269,701, 6,140,940, 6,014,100, 5,959,372, 5,742,225, 5,672,975, 5,535,243, 5,416,723, 5,207,101, 5,068,592, 5,065,152, 4,926,340, 4,656,353, 4,317,116, 3,764,880, EP-A 1 147 463, EP-A 1 058 093, EP-A 591 926, EP-A 525 920, EP-A 415,655, DE-A 44 12 388, or DE-A 39 34 007. In certain cases, there is provided within the field device electronics, as described, for example, in U.S. Pat. No. 3,764,880 or WO-A 04/048905, also a galvanic separation, for example between the internal operating and evaluating circuit, on the one hand, and the current controller, on the other hand, in order to prevent that possible potential differences, which are not always avoidable with certainty, between the plant, into which the field device is installed, and the external, electrical energy supply, are dropped uncontrolled.
For historical reasons, such two-wire field devices are, for the most part, so designed that a supply current instantaneously flowing in the single-pair line in the form of a current loop at an instantaneous electrical current level set at a value lying between 4 mA and 20 mA (=milliamperes), at the same time, also represents the measured value produced by the field device at that instant, or the actuating value instantaneously being sent to the field device, as the case may be. As a result of this, a special problem of such two-wire field devices is that the electric power at least nominally dissipatable or to be dissipated by the field-device electronics—in the following referenced in short as “available power”—can fluctuate during operation in practically unpredictable manner over a wide range. To accommodate this, modern two-wire field devices (2L, or two line, field devices), especially modern two-wire measuring devices (2L measuring devices) with (4 mA to 20 mA)-current loops, are, therefore, usually so designed that their device functionality implemented by means of a microcomputer provided in the evaluating and operating circuit is variable, and, to this extent, the operating and evaluating circuit, which, for the most part, does not dissipate much power anyway, can be adapted to the instantaneously available power.
A suitable adapting of the field-device electronics to the available power can e.g., as also proposed in U.S. Pat. No. 6,799,476, U.S. Pat. No. 6,512,358, or U.S. Pat. No. 5,416,723, be achieved by matching the power instantaneously dissipated in the field device to the instantaneously available power, and, indeed, in a manner such that individual functional units of the operating and evaluating circuit are operated with appropriately variable clock speeds, or, depending on the level of the instantaneously available power, even turned off for a period of time (ready, or sleep, mode). In the case of field devices embodied as two-wire measuring devices with active measurement pickup, the electric power instantaneously dissipated in the field device can, as disclosed in, among others, U.S. Pat. No. 6,799,476, U.S. Pat. No. 6,014,100, or WO-A 02/103327, additionally be matched to the instantaneously available power by adapting also the electric power instantaneously dissipated in the measurement pickup, for example by clocking of the, as required, buffered driving signal, along with a corresponding matchable strobe rate, with which the driving signal is clocked, and/or by reducing a maximum electrical current level and/or a maximum voltage level of the driving signal.
However, in the case of field devices embodied as two-wire measuring devices, a varying of the device functionality has, for the most part, the result that, during operation, an accuracy, with which the operating and evaluating circuit determines the measured value, and/or a frequency, with which the operating and evaluating circuit, for example, updates the measured value, are/is subject to changes in the instantaneously available power. Also the buffering of excess power present at times can only conditionally remedy this disadvantage of two-wire measuring devices with (4 mA to 20 mA)-current loops. On the one hand, due to the intrinsic explosion safety often required for such two-wire measuring devices, at best, existing excess electrical energy can be stored in only very limited amounts internally in the field-device electronics. On the other hand, however, the instantaneous supply current, and, to such extent, also the, at best, available excess energy, depends only on the instantaneous measured value, so that, in the case of a lastingly very low, but, timewise, strongly varying, measured value, a correspondingly provided energy buffer can, over a longer period of time, become completely discharged. Moreover, for establishing such a complex power management in the field device, a very comprehensive and, thus, also very demanding power measurement is required, both with respect to circuitry and with respect to energy; compare, in this connection, also WO-A 00/26739, U.S. Pat. No. 6,799,476, U.S. Pat. No. 6,512,358, or EP-A1 174 841
Apart from this, it has been found, in the case of field devices of the described kind having a measurement pickup for the conveying and measuring of media flowing at least at times, that the adaptive clocking of driving signals and/or of individual components of the operating and evaluating circuit is only conditionally suitable. This is true, especially in the application of a vibration-type measurement pickup, such as described, for example, in the above-mentioned U.S. Pat. No. 6,799,476, 6,691,583, 6,006,609, 5,796,011, 5,687,100, 5,359,881, 4,768,384, 4,524,610, or WO-A 02/103327. The field devices disclosed there serve to measure parameters of media flowing in pipelines, mainly mass flow rate, density or viscosity. To this end, the corresponding measurement pickup will include at least one measuring tube vibrating during operation and serving for the conveying of the medium, an exciter mechanism electrically connected with the field-device electronics and having an oscillation exciter mechanically interacting with the measuring tube for driving the measuring tube, as well as a sensor arrangement, which generates measurement signals by means of at least one oscillation sensor arranged on the measuring tube, for locally representing the measuring tube oscillations. Both the oscillation exciter and the oscillation sensor are, in such case, preferably of the electrodynamic type, thus constructed of a magnet coil and a plunging armature interacting therewith via a magnetic field.
Due to the highly accurate amplitude and frequency control of the exciter mechanism driving signal required for the operation of such a measurement pickup, unavoidable, for one thing, is a timewise high-resolution sampling of the measuring tube oscillations. Equally, in the case of measurements made on flowing media, the issued measured value must also itself be updated often. On the other hand, a, most often, very high mechanical time constant of the oscillation system formed by the measurement pickup leads to the fact that, in the case of possible accelerations of the same, especially during non-stationary, transient happenings, a high driving power must be used and/or relatively long settling times assessed. Further studies in this connection have, however, additionally shown that, because of the usually limited storage capacity for electric power, even a buffering of excess energy in the field device scarcely leads to any significant improvement of the signal-to-noise ratio dependent on the amplitude of the measuring tube oscillations. In this respect, even a temporary and partial switching-off of the operating and evaluating circuit is little suited for two-wire measuring devices with active measurement pickup of the described kind, especially for two-wire measuring devices having a vibration-type measurement pickup involving the conveyance of flowing media.
A further possibility for improving the power capability of field devices of the described kind, especially two-wire measuring devices, is, at least in the case of minimal available power, to use as much thereof as possible actually for the implementing of the device functionality, thus to optimize a corresponding efficiency of the field device, at least in the region of small available power. Supply circuits for the internal supply of the field electronics built on this principle are discussed in detail, for example, in U.S. Pat. No. 6,577,989, or U.S. Pat. No. 6,140,940. Especially, the solutions proposed therein aim to optimize the internally actually dissipatable, electrical power. For this purpose, provided at the input of the field-device electronics, for adjusting and maintaining the above-mentioned, internal input voltage of the field-device electronics at a predeterminable, as required also adjustable, voltage level, is an input voltage controller with a voltage adjuster, which acts at the input of the field device electronics and which, as a function of the instantaneously available power and an instantaneously actually needed power, has flowing through it, at least at times, a variable electrical current component branched from the supply current. However, a disadvantage of this field-device electronics is that all internal consumers are supplied practically from one and the same internal useful voltage and a possible collapse of this single useful voltage, for instance because of too little supply current, can lead to a state in which normal operation of the field device is no longer possible, or even to an abrupt, temporary, total stoppage of the field-device electronics.